The present invention relates to a method for improving the power rating and efficiency of a steam and gas turbine ("STAG") combined cycle plant. In particular, the invention concerns a method for increasing the total output power of a combined cycle plant, with no degradation in thermal efficiency, by cooling the air inlet to the gas turbine using a multi-component working fluid (such as a mixture of ammonia and water) and a portion of the waste heat in the low pressure section of a heat recovery steam generator ("HRSG") in the bottoming cycle. As discussed herein, a "chiller" positioned outside the HRSG uses a multi-component working fluid to cool the air inlet to the gas turbine to thereby improve the total output power of the turbine particularly when the ambient air temperature of the feed to the gas turbine exceeds about 60.degree. F.
"Combined cycle" power generation systems are well known in the art and typically include a gas turbine and a heat recovery steam generator that utilizes the heat from the gas turbine exhaust in order to generate high and intermediate pressure steam which in turn drives a steam turbine. The steam cycles used in conventional heat recovery steam generators vary considerably in design and operation. Typically, however, such systems use single-pressure or multiple-pressure heat recovery cycles (i.e., cycles that vary in steam pressure from high to low) with multiple heat recovery evaporators (boilers) to generate steam at the pressure levels necessary to drive different stages of the steam turbine.
The term "bottoming cycle" has long been used in the industry to describe the heat recovery steps which generate intermediate and low pressure steam for use in corresponding intermediate and low pressure stages of the steam turbine. In most combined cycle power plants, the inherent mismatch in thermal characteristics of the gas turbine exhaust (which releases sensible heat upon cooling in the HRSG) and the steam generator (which absorbs latent heat of boiling at a constant temperature) often results in thermodynamic losses in the range of about 10% of the work produced by the bottoming cycle.
A continuing need exists to improve the efficiency of conventional gas turbine combined cycle power plants. Thus, it has become common practice to utilize multiple pressure steam cycles to minimize the extent of the thermodynamic losses, particularly in the bottoming cycle. However, practical constraints limit the number of different pressures to three or less. With single pressure steam cycles, the thermodynamic losses can be higher than the 10% level. Even in multiple-pressure designs, considerable attention must be paid to optimizing the design of feed water preheaters, superheaters and reheater devices in order to minimize the potential efficiency losses.
The most common thermodynamic cycle for producing energy from a heat source in an HRSG is known as the "Rankine cycle." In a conventional Rankine cycle, a single working fluid (normally water) is evaporated using a readily available heat source and then expanded across one or more stages of the steam turbine, thereby transforming its energy into a more useable form for generating electricity. The "spent" working fluid vapor is then condensed in a condenser using an available cooling medium, such as cooling water. The potential energy of the condensed working fluid is increased by pumping it to a higher pressure and then re-heating the pressurized liquid in the HRSG to generate additional higher pressure steam as a new working fluid. Although the Rankine cycle works very effectively, it often suffers from efficiency losses due to the additional energy required to regenerate the steam, particularly in multiple-stage HRSG systems.
Another known problem in the operation of conventional combined gas/steam turbine cycles is that they experience reductions in total output power as compared to the rated value of the system when the air inlet temperature to the first stage gas turbine exceeds about 60.degree. F. during, for example, the hottest months of the year. In the past, various attempts have been made to use mechanical refrigeration to reduce the temperature of the inlet air to the gas turbine. One obvious drawback to conventional refrigeration systems is that they ultimately reduce the efficiency of the overall process because of the power necessarily consumed by the refrigeration compressor and related equipment.
During the past ten years, various patents have issued describing the use of multi-component working fluids (such as ammonia and water) to improve the efficiency of conventional Rankine cycles by substituting the multi-component fluid for water in the heat recovery steam generation cycles. These new systems--known generally as "Exergy systems" --operate on the general principal that a binary (multi-component) working fluid can be pumped to a high working pressure and then heated to partially vaporize the working fluid. The mixture is then flashed under non-isothermal conditions to separate the high and low boiling working fluid compounds, with the low boiling component being expanded across a turbine to drive the turbine and generate additional electricity. The high boiling component contains recoverable heat for use in heating the binary working fluid to evaporation. Typically, the high boiling component is then mixed with the "spent" low boiling fluid to absorb the spent working fluid in a condenser in the presence of a cooling medium.
In certain instances, these known Exergy cycles using multi-component fluids have demonstrated improved efficiencies as compared to Rankine cycles when a relatively low temperature heat source is employed. Multi-component systems, however, tend to provide less theoretical and practical advantages over conventional cycles when higher temperature heat sources are involved. Some later-generation multi-component systems provide improved efficiencies by using a distillation step in which part of the working fluid is distilled to assist in regenerating the working fluid. In that regard, various systems (known generally as "Kalina cycles") have been proposed as modifications of the original concept of using a multi-component working fluid to improve the thermodynamic efficiencies of bottoming cycles. See., e.g., U.S. Pat. Nos. 4,346,561; 4,489,563; 4,548,043; 4,586,340; 4,604,867; 4,732,005; 4,763480; 4,899,545; 4,982,568; 5,029,444; 5,095,708 and 5,203,899. One clear disadvantage of the prior art Kalina-type systems is that the multi-component "working fluid" can cause significant increases in overall turbine cost due to the inherent corrosive potential of ammonia-water working fluids on turbine blades and other components used in the power generation cycles.
In addition, although the modified Kalina bottoming cycle designs tend to reduce the thermodynamic losses in the HSRG by using mixtures that undergo non-isothermal changes in state, such systems must carefully match the thermal characteristics of the working fluid to that of the gas turbine exhaust by using multiple reheats and/or by partitioning the two-phase heat load on the system. Thus, in the past, it has been very difficult to incorporate the potential efficiency advantages offered by a Kalina-type cycle into a conventional multi-pressure steam bottoming cycle. Significant practical difficulties also exist in combining a multi-component working fluid cycle into a single component system because of the inherent differences in process conditions and materials used to operate the two cycles in tandem.
Accordingly, there is a need to provide a modification to a conventional bottoming cycle by adding a separate, multi-component cycle which uses a portion of the bottoming cycle waste heat but operates external to the HRSG as a "chiller" to cool the inlet air to the gas turbine.
There is a further need to provide for an improved bottoming cycle that incorporates a multi-component working fluid in one section of the bottoming cycle to enhance the heat recovery and reduce thermodynamic losses in the bottoming cycle, while at the same time preserving the advantages of maintaining a steam working fluid to drive the HRSG steam turbine.
Yet a further need is to modify the low pressure section of an HRSG that heats the water before it enters the low pressure boiler (sometimes called the low pressure "economizer" section of the HRSG) by using a portion of the economizer waste heat to operate an ammonia/water cycle as described in greater detail below.
Still yet another need is to modify a conventional combined STAG cycle by incorporating an ammonia-water cycle to take advantage of the inherent thermal mismatch between the exhaust gases and the water through the use of a dual component working fluid, and thereby improve the overall performance rating of the bottoming cycle, particularly when the air temperature into the first stage gas turbine exceeds about 60.degree. F. Lastly, there is a need to utilize an ammonia-water cycle to eliminate the need for supplemental cooling equipment (e.g., conventional mechanical refrigeration) in order to lower the ambient air inlet temperature to the first stage gas turbine.